Concern about the deadly new Middle East Respiratory Syndrome (MERS) virus has "significantly increased" but the disease does not yet constitute a global public health emergency, the World Health Organisation (WHO) said on Wednesday.

The virus, which causes coughing, fever and sometimes fatal pneumonia, has been reported in more than 500 patients in Saudi Arabia alone and has spread to neighbouring countries and in a few cases, to Europe and Asia. It kills about 30 percent of those who are infected.

The WHO's emergency committee, which met for five hours in Geneva on Tuesday, said on Wednesday that based on current information, the seriousness of the situation had increased in terms of public health impact, but that there is no evidence of sustained human-to-human transmission of the virus.

"The committee concluded that the conditions for a Public Health Emergency of International Concern (PHEIC) have not yet been met," the WHO said in a statement.

Global health regulations define such an emergency as an extraordinary event that poses a risk to other WHO member states through the international spread of disease, and which may require a coordinated international response.

The United Nations health agency stressed the need for countries where MERS cases are common to take immediate action to improve infection prevention and control, to try to halt the spread of the virus.

"This is most urgent for affected countries," it said, adding that key scientific investigations should be started as soon as possible to better understand the risk factors behind the disease. These include case-control, serological, environmental and animal studies.

The WHO's assistant director general for health security, Keiji Fukuda, said the main reason for not declaring MERS an emergency was that despite a surge in cases, the evidence did not suggest it was passing more easily from person to person.

"There is no convincing evidence right now for an increase in the transmissibility of this virus," he told reporters on a telephone briefing.

MERS is a virus from the same family as SARS, or Severe Acute Respiratory Syndrome, which killed around 800 people worldwide after it first appeared in China in 2002.

International fear about MERS has grown in recent weeks with a surge in cases detected in Saudi Arabia, where 495 people have been infected - 152 of whom have died - since the virus first emerged in 2012.

News in the past fortnight that the disease had reached the United States, imported via two healthcare workers who had travelled from Saudi Arabia, added to global concern.

On Tuesday, US health officials said two health workers based at a Florida hospital who had been exposed to a patient with MERS had begun showing flu-like symptoms.